Fischer-Tropsch synthesis, a process for the production of hydrocarbons from carbon monoxide and hydrogen, or synthesis gas, is well documented in the technical and patent literature. Fischer-Tropsch processes have also been commerically used, and are in operation today in some parts of the world.
The earlier Fischer-Tropsch catalysts were constituted for the most part of non-noble metals dispersed throughout a porous inorganic oxide support. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates, principally alumina. Most commercial experience, however, has been based on cobalt and iron catalysts. The first commercial Fisher-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. The cobalt and iron catalysts were formed by compositing the metal throughout an inorganic oxide support. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co:18 ThO.sub.2 :100 kieselguhr, parts by weight, and over the next few years by catalysts constituted of 100 Co:18 ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200 kieselguhr, respectively. These early cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughout. The iron catalysts, on the other hand, are not really suitable for natural gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: EQU H.sub.2 +2CO (CH.sub.2).sub.x +CO.sub.2 ;
with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: EQU 2H.sub.2 +CO (CH.sub.2).sub.x +H.sub.2 O.
Considerable effort has been expended in recent years to improve cobalt catalysts. For example, U.S. Pat. No. 4,542,122 by Payne et al, which issued Sept. 17, 1985, describes improved cobalt catalyst compositions useful for the preparation of liquid hydrocarbons from synthesis gas. These catalyst compositions are characterized, in particular, as cobalt-titania or thoria promoted cobalt-titania, wherein cobalt, or cobalt and thoria, is composited or dispersed upon titania, or titania-containing support, especially a high rutile content titania. U.S. Pat. No.4,568,663 by Mauldin, which issued Feb. 4, 1986, also discloses cobalt-titania catalysts to which rhenium is added to improve catalyst activity, and regeneration stability. These catalysts have performed admirably well in conducting Fischer-Tropsch reactions, and in contrast to earlier cobalt catalysts provide high liquid hydrocarbon selectivities, with relatively low methane formation.
These and other recently developed forms of cobalt-titania catalysts offer promise of a viable modern day large scale commercial Fischer-Tropsch plant which may utilize such catalyst, particularly catalysts formed by dispersion in one form or another of Co-Re, Co-Hf and Co-Ce on a rutile form of titania. Despite the admirably high activity and selectivity of these catalysts, however, there nonetheless remains a need for further improvements in the activity, selectivity and productivity of Fischer-Tropsch catalysts, notably cobalt catalysts. Productivity, which is defined as the standard volumes of carbon monoxide converted/volume catalyst/hour, is, of course, the life blood of a commercial operation. High productivities are essential in achieving commercially viable operations. However, it is also essential that high productivities be achieved without high methane formation, for methane production results in lower production of liquid hydrocarbons.